Assetic has several filters, but includes no external libraries out of the box. You'll need to get CSSMin http://code.google.com/p/cssmin and integrate it into your project, much as you probably did with Gaufrette, Imagine, BaseFacebook for FOSFacebookBundle, or other similar ones (even Assetic counts) etc.

In case you don't know what I'm talking about, just do the following:- get cssmin-v3.0.1.php (the latest version) from the link I posted- rename it to cssmin.php and put it in vendor/cssmin/src inside your project- Check out your autoload.php, add the following line in the list of registered namespaces: